Stratospheric Water Vapor Simulations: From Polar Regions to the Tropical Tropopause

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Water vapor is the most important contributor to the natural greenhouse effect. Changes in stratospheric water vapor concentrations and its distribution can affect the Earth's radiative budget and surface temperatures as well as stratospheric chemistry. Water is also an essential constituent of polar stratospheric clouds (PSCs), which play a key role in polar ozone destruction. In addition, cirrus clouds in the lowermost stratosphere (LMS) may have important chemical and radiative effects, but little is known about their formation mechanisms. At the same time, the region close to the tropopause is particularly sensitive to even small changes in water vapor and an in-depth understanding of cloud microphysical processes in this region is an important prerequisite for reliable climate predictions.Within the framework of the proposal, "Stratospheric Water Vapor Simulations: From Polar Regions to the Tropical Tropopause" (i) freeze-drying processes in the tropical tropopause layer (TTL), (ii) mid-latitude cirrus clouds in the LMS, and (iii) PSCs and their impact on ozone destruction will be examined in the context of an improved representation in global simulations. A variety of new measurements and studies on PSCs has challenged the conventional understanding of formation pathways. Different nucleation pathways shall be quantified in terms of their potential for chlorine activation to achieve more refined forecasts for the recovery of the ozone layer. Moreover, a main goal of this project is research on stratospheric cirrus clouds that form both in the mid-latitudes and in the TTL. The contribution of cirrus clouds to the global radiation budget is highly dependent on the size distribution and therefore on the nucleation rate of ice particles and remains an important source of uncertainty in global climate predictions. Furthermore, it is essential to understand dehydration of ascending air masses through the TTL. The Chemical Lagrangian Model of the Stratosphere (CLaMS), developed at the Forschungszentrum Jülich, allows a precise simulation of dehydration and denitrification, both processes influencing ozone depletion and changes in the stratospheric composition. In combination with high-resolution in-situ measurements and global satellite data, the microphysics of stratospheric ice clouds will be investigated in a unique way. Coupling the Lagrangian concept of CLaMS with the climate model EMAC enables one to investigate the effect of changes in stratospheric water vapor concentrations on surface temperatures. Therefore, the project contributes to an improved understanding of key processes, which determine inter-annual variability and trends in stratospheric water vapor and drive decadal climate variability.